Field of the Invention
The present invention relates to a fan and a water heater provided with the fan, and an impeller and a water heater provided with the impeller.
Description of the Background Art
In replacement of an already placed tank water heater with an instantaneous water heater, there are locations where an already placed exhaust pipe (a B vent) cannot be removed from a point of view of maintaining appearance of buildings.
At such a location, a water heater can be replaced by leaving the already placed exhaust pipe and inserting an exhaust tube (a flexible exhaust tube) in the exhaust pipe. The exhaust tube should be smaller in diameter, because the exhaust tube cannot be placed in the exhaust pipe if the exhaust tube has a large outer diameter. In order to maintain a stable combustion state even when the exhaust tube is decreased in diameter, an exhaust suction and combustion type should be adopted for a water heater.
A water heater of this exhaust suction and combustion type is disclosed, for example, in Japanese Patent Laying-Open No. 60-186617. In the water heater described in this publication, a heat exchanger for recovering sensible heat, a heat exchanger for recovering latent heat, and a fan are arranged in this order on a downstream side in a flow of a combustion gas produced in a burner. Namely, in the water heater of this type, the fan is arranged downstream of the heat exchanger in the flow of the combustion gas, and the fan suctions combustion gas having passed through the heat exchanger and emits the combustion gas to the outside of the water heater.
In addition, an impeller having a plurality of blades around the rotation shaft is known as a component of the fan (for example, Japanese Patent Laying-Open No. 2000-356197, Japanese Patent Laying-Open No. 2010-242543, Japanese Patent Laying-Open No. 2010-281256, U.S. published patent application No. 2008/0279682 (specification), and Japanese Utility Model Laying-Open No. 04-040191). This impeller is driven by a motor or the like and rotated, thereby achieving the air-blowing function of the fan.
Such an impeller is formed of a plurality of components. There is a known method for producing the impeller by ultrasonic-welding resin components such as a blade to each other for assembly.
Ultrasonic welding is a processing technique for instantaneously melting and joining a thermoplastic resin by minute ultrasonic vibration and welding pressure. According to the ultrasonic welding, a portion welded by ultrasonic waves is generally made of a material that is once melted and thus becomes brittle. Consequently, this portion may often be lower in durability and strength than the base material. Furthermore, when welding is insufficient, durability and strength may be further lowered.
Therefore, when an impeller is produced by means of welding such as ultrasonic welding, sufficient welding of the weld portion is required in order to maintain the durability and strength of the impeller.
Furthermore, it is also conceivable to use a resin containing a fibrous filler in order to increase the strength of the impeller. However, it has been found that, in the case where a resin containing a fibrous filler is used to produce a resin component including a disc-shaped (or annular) main plate and a plurality of blades each extending in the radial direction of the main plate, a distortion occurs in the shape of the resin component, particularly in the shape of the main plate in the radial direction, with the result that the flatness of the main plate tends to decrease.
When a resin component having a main plate with decreased flatness is ultrasonic-welded to another component, these components cannot be equally pressurized, so that ultrasonic vibration cannot be stably transmitted. Consequently, welding of these components tends to be insufficient. Furthermore, when a distortion occurs in the shape of the main plate of the impeller, the motion balance performance of the impeller may be decreased, so that the air-blowing performance may be decreased.
Particularly, the impeller of the fan used for a water heater of an exhaust suction and combustion type is placed within a passage of combustion gas, and therefore, exposed to a higher temperature environment as compared with the conventional case. Furthermore, the impeller of the fan used for a water heater of a latent heat recovery type adapted to an exhaust suction and combustion system is exposed also to strong-acid drainage water produced by recovery of latent heat, in addition to a high-temperature environment. In such a case, it is strongly required to sufficiently weld each component to particularly maintain the durability of the impeller.